Getter devices can be used in all the technological and scientific applications wherein vacuum maintenance is required, such as, for example, flat displays (of the plasma or field emission type), some kind of lamps or particle accelerators for scientific research. Another important field of use of the getter devices is gas purification, inside fluorescent lamps but mainly in the case of the process gases for microelectronic industry.
The active materials which form these getter devices are mainly zirconium and titanium and alloys thereof with one or more elements selected from among the transition elements and aluminum. Such materials have a strong ability to absorb gases of low molecular weight, such as oxygen, water, hydrogen, carbon oxides and in some cases nitrogen, and therefore are used for removing traces of these gases from spaces where the vacuum must be maintained, or remove such gases from atmospheres or flows of gases which are inert towards these materials, mainly noble gases.
Since gas sorption takes place through the surface of the getter material, it is generally preferable that such as surface as wide as possible. In order to obtain this result, while maintaining small device size, porous devices are generally used. The porous devices are formed of consolidated powders of getter materials which allow a high ratio of exposed surface of active material to the geometrical surface of the getter device.
Various methods for manufacturing porous getters devices are described in the literature. For example, Great Britain patent GB-B-2,077,487 describes the production of porous getter devices formed of a mixture of powders of a getter metal, particularly titanium or zirconium, with a getter alloy; the mixture is precompressed and sintered in a vacuum oven at temperatures between approximately 800 and 1100° C. The getter alloy, which has a sintering temperature higher than that of the metal, is added to provide the antisintering function, in order to avoid an excessive compaction of the powders with following reduction of the gas sorption features. Patent application DE-A-2,204,714 discloses a process similar to that of the cited patent GB-B-2,077,487, with the difference that in this case graphite powder is used as an antisintering agent.
Getter devices having a porosity degree higher than those obtained by the two previously described techniques can be manufactured by the electrophoretic technique, described in U.S. Pat. No. 5,242,559 to Ettore, which is incorporated herein by reference in its entirety. According to this technique, a suspension, generally hydroalcoholic, of particles of a getter material is prepared. In the suspension are inserted two electrodes one of which, made of metal or graphite, will also serve as a support of the final getter device. The transport of the getter material particles towards the support and their adhesion thereon is caused by applying a potential voltage difference between the two electrodes. The deposit obtained is then consolidated by a sintering thermal treatment in a vacuum oven, generally at temperatures ranging between about 900 and 1000° C.
Getter devices wherein the active material is in the form of a layer on a planar support can be produced by the screen-printing technique, which is described in U.S. Pat. No. 5,882,727, to Corazza, et. al., and which is herein incorporated by reference in its entirety. According to this screen-printing technique, a paste of getter material particles is prepared in an aqueous solution containing low percentages of an organic compound having a high boiling point, which acts as a binder. This paste is passed through the meshes of a suitable net, and is deposited on the underlying substrate. The deposit is then dried and consolidated by sintering in a vacuum oven at a temperature between about 800 and 1000° C.
Finally, getter devices having a particularly high porosity degree can be obtained according to the technique described in U.S. Pat. No. 5,908,579 to Conte, et. al., which is herein incorporated by reference in its entirety. The technique taught in this patent uses a mixture of powders of the getter material and of an organic component, for example ammonium carbamate. The organic component evaporates during the thermal treatments of the consolidation of the getter device. Such treatments generally reach temperatures between 900 and 1200° C., and leave a net of interconnected porosities or micropassages which allow the access of gases to the surface of the innermost particles of getter material in the device.
A problem encountered with the getter devices according to the above-described known art is the possibility of the loss of particles due to the fact that the surface particles of the getter tend to be bound more weakly than the internal particles. The presence of free particles is painful for most of the anticipated applications of the getter devices, because such free particles may interfere with the functionality of the device. One example is the case of flat displays. In other examples, such freed particles may come between the path of radiations or elementary particle beams (such as applications in particle accelerators) or the free particles may deposit on microelectronic devices which are being manufactured.
A possible solution to this problem is to increase the sintering temperature, thus favoring the mutual adhesion of the particles. However, this method not only reduces the entity of the particle loss problem without solving it, but also has the disadvantage that it leads to a reduction of the porosity and of the exposed area of the active material, which results in a reduction of the gas sorption properties of the getter devices.
What is needed is a method by which porous getter devices can be manufactured increasing the adhesion of particles without increasing the sintering temperature and the associated negative effects in the reduction of porosity.